1. Field of the Invention
This invention relates to a pigment dispersion, a precursor of ink for UV-curing type ink-jet recording, a method of ink-jet recording, a printed matter, and a method of manufacturing a pigment dispersion.
2. Description of the Related Art
In a situation where a fairly large number of copies of printed matter are required such as in the cases of printing copies of local advertisement bill and enterprise handouts, a printing machine utilizing a form plate has been conventionally employed for meeting such requirements. In recent years however, an on-demand printer which is capable of quickly coping with the diversification of needs and of minimizing stocks has been increasingly utilized in place of the aforementioned conventional printing machine. As for such an on-demand printer, an electrophotographic printer where a toner or a liquid toner is employed as well as an ink-jet printer which is capable of achieving high-speed and high-quality printing are expected to be useful.
It is known that, as in the case of the printing machine utilizing a form plate, the on-demand printer is designed to employ a solvent type ink or a solvent type liquid toner both containing a pigment and an organic solvent. This technique of employing a solvent type ink or a solvent type liquid toner is however accompanied by a problem that the organic solvent volatilizes to such an extent that cannot be disregarded when the printing is repeated a certain number of times. Therefore, when this technique is employed, a problem of environmental contamination due to this volatilized organic solvent will be raised, thus necessitating the provision of a strict exhaust gas processing installation or a strict solvent recovery system.
On the other hand, in the case of the ink-jet printer, a solvent type ink can be handled in a closed system until the ink is discharged onto the surface of printing matter. Moreover, since the ink can be employed without substantially generating redundant ink, it is possible, through the selection of ink which is relatively safe or through the provision of suitable countermeasure for the exhaustion of solvent, to alleviate the aforementioned problem of environmental contamination.
Further, a photosensitive ink for ink-jet recording (ink-jet ink) as well as a printer system employing the photosensitive ink are now being taken notice of. This technique is characterized in that the photosensitive ink that has been delivered onto the surface of printing matter is caused to quickly photocure. A representative example of this photosensitive ink is formed of a radical polymeric monomer, a photopolymerization initiator and a pigment. There is also proposed a photopolymerizable photosensitive ink which comprises a cation polymeric monomer, a cation-generating agent and a pigment. According to these techniques, since an ink layer formed using such a photosensitive ink can be readily nonfluidized by the irradiation of light, it is possible to obtain a printed matter which is unharmful and of high-quality.
On the other hand, in the case of the ink-jet recording apparatus which is adapted to be employed in such a quick-drying printing as described above, the printing heads are generally linearly arrayed for accelerating the printing, thus making it possible to perform the printing with one pass over a printing surface. Therefore, any error in the delivery of ink would become a defective printing (missing of printing) on a line of the printed surface. Therefore, it is indispensable to make such an error zero as much as possible. Namely, it is required for an ink to be employed in the ink-jet recording that the ink is made up of materials which are excellent in stability enabling the ink to achieve very high printing accuracy and delivery stability.
The aforementioned ink of cation polymeric type has been proposed for enhancing the adhesion of the radical polymeric type ink as well as for modifying the low sensitivity, due to oxygen inhibition, of the radical polymeric type ink, so that the demand for this cation polymeric type ink is expected to increase in future. However, these photo-curing type inks for ink-jet recording are accompanied with a problem that due to high reactivity, the physical properties such as viscosity of these inks readily fluctuate to a great extent, thus making these inks inherently unstable. The reason for this can be attributed to the fact that once active species is generated on account of some sort of factor (such as heat for instance), it is difficult to easily deactivate the active species, thereby frequently allowing the dark reaction to take place in the ink. Once the viscosity, surface tension, elastic force, etc., of the ink changes on account of the dark reaction, it is more likely that the ink can be brought into a critical state such as the turbulence in trajectory configuration of ink, the deterioration of printing reproducibility, or in the worst case, the failure of delivery of ink, the clogging of ink, etc. Therefore, this problem is very serious.
Even if the aforementioned problem can be overcome, these photo-curing type inks are still accompanied with a problem that printing error may occur though the possibility of generating the printing error may be low. It is now made clear that the state of ink should be stabilized not only macrostructurally but also microstructurally. This kind of ink contains, in addition to a solvent, a pigment having quite a different physical property, and a large quantity of resinous components such as a dispersing agent. Since this kind of ink is made up of a mixture, the state of dispersion in the ink may change spontaneously as exemplified by the aggregation of pigment. This change of the state of dispersion will be further accelerated due to the aforementioned dark reaction, thus leaving a problem which the conventional photosensitive ink compositions or the conventional manufacturing methods of ink have failed to overcome.